Method of manufacturing a piezoelectric actuator

ABSTRACT

A method for making a piezoelectric actuator comprises coating at least one of a first surface and a second surface of a piezoelectric element with a polyimide adhesive. The piezoelectric element is then heated to dry the adhesive. Afterwards, the piezoelectric element is inserted between a first metallic layer and a second metallic layer to form an assembly. The assembly is placed in a press. While the assembly is in the press, the polyimide adhesive is cured at a curing temperature which does not depole the piezoelectric element, thereby bonding the piezoelectric element between the first metallic layer and the second metallic layer.

This application is a national stage of international applicationPCT/US01/28947 filed 14 Sep. 2001 which designates the U.S., and whichclaims the benefit and priority of U.S. Provisional Application60/233,248, filed Sep.18, 2000, all of which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention is in the field of the manufacture offerroelectric actuators and miniature diaphragm pumps using theseactuators as the prime mover. In the best mode the actuators arepiezoelectric.

BACKGROUND ART

The prior art for this invention may be grouped as follows:

-   I. U.S. Pat. Nos. 5,471,721, 5,632,841, 5,849,125, 6,162,313,    6,042,345, 6,060,811, and 6,071,087 showing either prestressing of    piezoelectric actuators, or dome-shaped piezoelectric actuators, or    both. This prior art is generally in apposite because the present    invention does not use a prestressed or dome-shaped piezoelectric    actuator.-   II. U.S. Pat. Nos. 6,179,584, 6,213,735, 5,271,724, 5,759,015,    5,876,187, 6,227,809 showing so-called micropumps. Such pumps    generally pump only a drop of fluid at a time; because of the small    forces and low Reynolds numbers involved, this prior art is    generally in apposite.-   III. U.S. Pat. Nos. 4,034,780, 4,095,615 showing flapper valves.    These are flappers mounted on a separate hinge. No prior art was    found showing a flex valve with a miniature pump.-   IV. U.S. Pat. Nos. 5,084,345, 4,859,530, 3,936,342, 5,049,421    showing use of polyimide adhesives for various purposes, including    bonding metals and other materials to film.-   V. U.S. Pat. Nos. 4,939,405, 5,945,768 showing electrical driver    circuits for piezoelectric actuators.-   VI. U.S. Pat. Nos. 6,227,824, 6,033,191, 6,109,889, German WO    87/07218 showing various kinds of pumps incorporating piezoelectric    actuators.

DISCLOSURE OF INVENTION

This invention is a method for making a high-displacement ferroelectricactuator, in this case a piezoelectric actuator. This piezoelectricactuator may then be used as the diaphragm in a small diaphragm pump.The pump is small, lightweight, quiet, and efficient. The best mode, around pump about 40 mm [1.5″] in] diameter by about 13 mm [0.5″] thickand weighing approximately 35 g [one ounce], can pump upwards of 450milliliters of water or other fluids per minute. These pumping rates areaccomplished using a six-volt battery at 25 ma driving through a smallelectronic driver circuit, approximately 25 mm [1″] square. This circuitforms part of the invention. The one way valve[s] necessary foroperation of the invention are flex valves in which a thin film ofpolyimide acts as the working element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the pump of the invention with the partsin the positions they would be for the best mode.

FIG. 2 is a sectional view of the pump along line 2—2 of FIG. 1.

FIG. 3 is a sectional view of the press used to make the piezoelectricactuators of the invention.

FIG. 4 shows the driver circuit for the piezoelectric actuator used withthe pump.

FIG. 5 is a partially diagrammatic view showing an alternativeembodiment of the invention in which the pump chamber is reduced insize.

FIG. 6 is a partially diagrammatic view showing another alternativeembodiment of a pump in which the inlet and outlet are perpendicular tothe plane of the actuator.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows how the piezoelectric actuator of the present invention maybe used in a miniature diaphragm pump. The pump 10 is generally in theform of a circular short cylinder. It includes the pump body 12,piezoelectric actuator 14, pump cover 16 and piezoelectric actuatorelectronic driver circuit 18. The pump body 12 has lugs 20 for mountingthe pump to any substrate. Inlet 22 and outlet 24 are part of the pumpbody 12 though they could be separate pieces otherwise fastened to thepump body. The pump cover 16 is essentially the same diameter and of thesame material as the pump body 12. The material would ordinarily be of astandard plastic such as acetal [DELRIN®], PVC, or PC, or of a metalsuch as stainless steel or brass. These are preferable since they can beeasily machined or thermally formed. The cover 16 may be fastened to thepump body 12 by any means such as by a fast-curing adhesive while thepump body 12 and cover 16 are under compression such as by clamping. Thepump cover has an opening 26 for venting the space above the actuator14.

The dimensions of the pump depend on the particular application. In thebest mode the pump body 12 is about 40 mm [1.5″] in diameter. A pumpchamber 30 is formed in the center of the pump body 12, for example bymolding or machining. The pump chamber 30 is about 28 mm [1.125″] indiameter or about 3 mm [⅛″] less in diameter than the diameter of thepiezoelectric actuator 14. The chamber 30 is about 6 mm [0.25″] deep. Aseat 32 about 3 mm [0.125″] wide and about 2 mm [0.070″] deep isprovided in the pump body 12 at the top of the pump chamber 30. As shownin FIG. 2 the piezoelectric actuator 14 is mounted on the seat 32 toform the diaphragm in the top of the pump chamber 30.

To assemble the pump a sealing washer 34 the same diameter as thepiezoelectric actuator is put on the seat 32 to seal the pump chamberwhen the piezoelectric actuator 14 is put in place. The sealing washer34 may be of a relatively soft material such as Buna-N or silicon rubberto account for any irregularities in the mating surfaces and ensure agood seal between the actuator 14 and the pump body 12. Once thepiezoelectric actuator 14 is in place an o-ring seal 36 is placed on topof the piezoelectric actuator 14 to hold the piezoelectric actuator 14in place and seal it from the cover 16. The cover 16 of the same outsidediameter as the pump body 12 base but only about ⅛″ thick is then put inplace. Sealing washer 34 and o-ring seal 36 are referred to collectivelyas the pump seals, even though they both have the additional function offixing the actuator 14 in place with respect to the pump body 12. Thecover 16 is then fastened to the body 12 while under compression, forexample by adhesive under clamping pressure, to seal the piezoelectricactuator 14 to the body 12 and fix the actuator 14 in place to allowpumping action.

The process for making the piezoelectric actuator 14 generally is asfollows:

A piezoelectric wafer 38 formed of a polycrystalline ferroelectricmaterial such as PZT5A available from Morgan Electro Ceramics isobtained. As the name implies this material is actually a ceramic. It isprocessed into the high displacement piezoelectric actuator 14 bylaminating the piezoelectric wafer 38 between a metal substrate layer 40and an outer metal layer 42 as shown in FIG. 2., where the thicknessesof the three layers and the adhesive between them are exaggerated forclarity. The bonding agent 41 between the layers 38 and 40 is apolyimide adhesive. This lamination process does several things: Itruggedizes the piezoelectric actuator 14 because the metal layers keepthe piezoelectric from fracturing during high displacement. It permitshigher voltage due to the relatively low dielectric constant of thepolyimide adhesive, thereby allowing 3–5 times higher displacement thana conventional piezoelectric. Being laminated between metal layers usinga high performance polyimide adhesive makes the piezoelectric actuatorhighly resistant to shock and vibrations. With this inventionpiezoelectric actuator devices can be used in environments as hot as acontinuous 200° C., compared to only 115° C. for a conventionalpiezoelectric. The significant increase in temperature is due to thepolyimide adhesive used in the bonding process which is unaffected bytemperatures up to 200° C. Epoxy adhesives used in conventionalpiezoelectrics normally can withstand temperatures up to only 115° C.This increase in operating temperature would allow the pumps of thisinvention to be used in a variety of pump applications, even pumpingboiling water continuously.

The piezoelectric wafers 38 are available from the vendor mentioned invarious shapes and thicknesses. For the invention circular wafers 25 mm[1″] in diameter and 0.2 mm [0.008″] thick were found to be optimum.Square wafers were tried but did not give maximum displacement. Ingeneral the thinner the wafer, the greater the displacement at a givenvoltage, but the lower the force. The 0.2 mm [8-mil] thickness gives thebest flow rate for the diameter of the wafer.

In the best mode stainless steel 0.1 mm [0.004″] thick is used for thesubstrate layer 40, the layer in contact with the pumped liquid.Stainless steel is chosen for its compatibility with many liquids,including water, its fatigue resistance, its electrical conductivity andits ready availability at low cost. Aluminum 0.05 mm [0.001″] thick isused for the outer layer 42 primarily for its electrical conductivity intransmitting the actuating voltage to the piezoelectric wafer 38 acrossits surface, but also for its robustness and ready availability at lowcost.

The diameter of the piezoelectric wafer 38 being about 25 mm [1″] asnoted above, the diameter of the substrate layer 40 is about 40 mm[1.25″]. The setback of the wafer 38 from the edge of the substratelayer 40 is an important feature of the invention. This leaves a rimthat serves as a clamping surface for the actuator assembly. This meansthat the entire piezoelectric wafer 38 is free and relativelyunconstrained, except insofar as it is bonded to the substrate 40 andthe outer layer 42. This allows maximum displacement of the actuator 14,ensuring maximum flow of liquid through the pump.

The diameter of the outer layer 42 is smaller than the diameter of thewafer 38. This setback of the outer layer 42 from the edge of the wafer38 is done to prevent arcing over of the driving voltage from the outerlayer 42 to the substrate layer 40.

Other materials and thicknesses may be used for the enclosing layers 40and 42 as long as they meet the requirements noted.

Of special note is that the piezoelectric actuator of the invention isflat. In much of the prior art the actuator is dome-shaped, it beingsupposed that this shape is necessary for maximum displacement of theactuator and therefore maximum capacity of the pump for a given sizeactuator. Special molds and methods are proliferated to produce theshapes of the actuator considered necessary, or to produce a prestressin the actuator that is supposed to increase its displacement. Our testsof the invention have shown, however, that a dome shape is notnecessary, and that the flat actuator has a higher pumping capacity fora given size than any known pump in the prior art. As such the actuatoris much simpler to produce in large quantities, as the following willdemonstrate. The flat shape also means that the pump may be smaller fora given application. A flat actuator is also inherently easier to mountin any given application than a dome shaped actuator would be.Furthermore, pumps using the actuator have been shown to havesufficiently long life for numerous applications. Many manufacturerswhose names are household words are using or testing this invention.

The process for making the piezoelectric actuator 14 specifically is asfollows:

-   1. The piezoelectric wafer 38 and and enclosing layers 40 and 42 are    cleaned using a solvent that does not leave a residue, such as    ethanol or acetone. All oil, grease, dust and fingerprints must be    removed to ensure a good bond.-   2. The piezoelectric wafer 38 is then coated on both sides with a    thin layer 41, not more than 0.1 mm [0.005″], of a high performance    polyimide gel adhesive such as that available from Ranbar Inc. The    gel should contain a minimum of 25% solids to allow sufficient    material for a good bond after the solvent is driven off-   3. The piezoelectric wafer 38 is then placed under a standard heat    lamp for about 5 minutes to remove most of the solvent from the gel    and start the polyimide gel polymerization process. Both sides of    the piezoelectric must be cured under the heat lamp since both sides    are to be bonded to metal.-   4. Once the adhesive is dry to the touch, the piezoelectric wafer 38    is then placed between the substrate layer 40 and the outer layer    42.-   5. The assembly is placed in a special press. This press was    developed specifically for making piezoelectric actuators 14 and    provides uniform temperature and pressure to ensure a good bond    between the three components of the actuator. Referring to the best    mode shown in FIG. 3 the press comprises two 300 mm [12″] square by    6 mm [¼″] thick plates of aluminum 101 held together with    thumbscrews 102, four on each edge. To ensure uniform pressure while    in the press, the bottom plate 101 of the press is covered with a    sheet of low cost polyimide film 104 such as Upilex available from    Ube Industries Ltd. The piezoelectric actuators 38 are placed on the    film and a sheet of high temperature, 4 mm [⅛″] thick rubber 106 is    placed over the piezoelectric actuators. The rubber on top and the    film on bottom cushion the piezoelectric actuators 38 providing even    distribution of pressure when the press is taken to temperature. Of    course other dimensions of the press plates are possible.-   6. Once the piezoelectric actuators are placed in the press the    thumbscrews 102 are made finger tight.-   7. The press is then placed in a standard convection oven for thirty    minutes at about 200° C.-   8. The press is removed from the oven, allowed to cool to a safe    temperature, and the actuators 14 removed from the press.

The press 100 is the result of an effort to develop a low cost, rapidprocess for manufacturing piezoelectric actuators. The press takesadvantage of the thermal expansion of the aluminum plates 101 whichcreates the necessary pressure to cause the polyimide adhesive to bondto the piezoelectric wafer 38 and metal layers 40, 42 while it is atcuring temperature. The press can be put into the oven, and taken out,while the oven is at temperature thereby allowing continuous operationduring the manufacturing process. The abrupt change in temperature doesnot affect the piezoelectric actuators 14 since they will remain underpressure even while the press is removed from the oven and allowed toassume room temperature.

Of special note is that this press process is one of further driving offthe solvent and curing the polyimide at a relatively low temperature.Prior art processes for making similar piezoelectric actuators requirethe mold/press to be taken to much higher temperatures, high enough tomelt the polyimide adhesive. Furthermore, since such high temperaturesdepole the piezoelectric ceramic, it is necessary to pole it again atthe end of the process. The present invention eliminates this stepaltogether, thus contributing to the lower cost of manufacturing thepiezoelectric actuators.

Using these simple methods and hardware it is possible to manufacturehundreds of thousands of piezoelectric actuators 14 per month, or evenmore, depending on the scale of the operation desired.

The principle of the piezoelectric actuator pump 10 is the same as forany diaphragm pump. Normally the diaphragm in a diaphragm pump isoperated by a cam or a pushrod connected to a motor or engine. This isnot the case in the piezoelectric actuator pump 10. The piezoelectricactuator 14 acts as the diaphragm and moves when a pulsed electric fieldis imposed across the piezoelectric wafer 38 by means of the enclosinglayers 40 and 42. This varying electric field causes the piezoelectricactuator 14 to expand and contract. As the actuator 14 expands, with itsedge constrained, it assumes a slight dome shape as the center of theactuator moves away from the pump chamber 30. This draws liquid into thepump chamber 30 through the inlet 22. When the piezoelectric actuator 14contracts it moves toward the liquid, forcing it out of the pump chamber30 through outlet 24.

One of the problems with prior art piezoelectric actuators has been thevoltage necessary to drive the piezoelectric. To provide power to thepiezoelectric actuator pump 10 the electrical driver 18 shown in FIG. 4was invented that converts the voltage from any six volt d.c. powersource to an alternating current of over 200 volts peak-to-peak. Thisvoltage is sufficient in the preferred embodiment to drive apiezoelectric actuator to attain the pumping rates noted above. In thecircuit in FIG. 4 point A is connected to the substrate layer 40 whilepoint B is connected to the outer layer 42.

Piezoelectric actuators perform better when the peak-to-peak voltage isnot evenly balanced. They respond better to a positive voltage than thesame negative voltage. Thus the circuit 18 has been designed to producealternating current with the voltage offset to 150 volts positive and 50volts negative. This is sufficient voltage for the piezoelectricactuator to make a very efficient pump. While a sinusoidal wave willwork, at the lower frequencies and voltages, a square wave makes thepiezoelectric more efficient. Values of the circuit components in FIG. 4are as follows:

R1 8 to 20 MΩ R2 8 to 20 MΩ R3  680 KΩ R4   1 MΩ C1  0.1 μF C2  0.1 μFC3  0.1 μF[200 v] C4 0.47 μF[200] L1  680 μH D1 BAS21 diode

U1 is an IMP 528 chip designated an electroluminescent lamp driver. Inthis circuit, with the other components, it serves to shape the pulsesand amplify them to the 200 volt peak-to-peak value needed to drive thepiezoelectric actuator 14. The values of R1 and R2 are chosen to varythe frequency of the output between about 35 Hz and about 85 Hz,depending on the particular application.

This circuit is composed of miniaturized components so it may becontained in a box 302 approximately 25 mm [1″] square by 6 mm [¼″]deep. It has only eleven off-the-shelf surface mount components. The box302 may be mounted anywhere in proximity to the pump 10. In the bestmode it is mounted on top of the pump, as shown in FIGS. 1 and 2, forexample by an appropriate adhesive. Leads 15 run from the driver circuit18 and are fastened to spring loaded contacts 304 such as those sold bythe ECT Company under the trademark POGO®. These contacts 304 aremounted in a box 306 on top of pump cover 16 and project through thepump cover 16 to make contact with the two layers 40 and 42. This smalldriver circuit eliminates the need for the large power supplies andtransformers used in prior art piezoelectric applications. Alternativelythe leads 15 could be run through an opening in the cover 16 andfastened electrically to the layers 40 and 42, as by soldering. O-ring36 is soft enough to accommodate the soldered point on the substratelayer 42.

Several conventional types of one-way valves were evaluated as inlet andoutlet valves for the piezoelectric actuator pump 10. All had variousdrawbacks including bulk and poor response to the dynamic behavior ofthe piezoelectric actuator 14. An inline flex valve 200 was inventedthat is well adapted to the action of the piezoelectric actuator 14 asshown in FIG. 2. The working element of the flex valve is an ellipticaldisk 202 of polyimide film about 0.05 mm [0.002″] thick. The disk 202 isthe same size and shape as the end of a short piece of rigid tube 204formed at about a 45° angle to the axis of the rigid tube 204. Theinside diameter of the rigid tube 204 is the same as the inside diameterof the inlet 22 or outlet 24 of the pump body 12. Rigid tube 204 iscaptured in the end of the flexible system conduit 206 which slips overthe inlet/outlet 22,24 and carries the system liquid, as shown in FIG.2. Valve disk 202 is attached to the nether end of the slanted surfaceat the point designated 203 by any sufficient means such as by adhesiveor thermal bonding. A similar flex valve 200 may be placed in the outlet24. Both disks 202 of both valves would point in the same directiondownstream. However, it was found in operating the pump 10 that it wouldpump at full capacity with no valve at all in the outlet. It ispostulated that the liquid in the inlet circuit, even with the inletvalve partially open, provides enough inertia to act as a closed inletvalve. Operation with only the inlet valve is considered to be the bestmode.

This flex valve 200 is an important feature of the invention. It is ofabsolute minimum bulk. The mass of the disk 202 is also about as lightas it could possibly be so it reacts rapidly to the action of theactuator 14. When it is open it presents virtually no resistance to thesystem flow. Mounted at the 45° angle, it has to move through an angleof only 45° to fully open, whereas if it were mounted perpendicular tothe flow it would have to move through an angle twice as large. It is ofextreme simplicity and low cost of materials and fabrication. Also nopart of the valve 200 projects into pump chamber 30. This minimizes thevolume of pump chamber 30 which helps make the pump self-priming andincreases its efficiency. Further contributing to these characteristicsis that the flex valve 200 is biased closed when the pump is notoperating.

FIGS. 5 and 6 show alternative embodiments of the pump of the invention.The pump in FIG. 5 is essentially the same as that of FIG. 2 except thatthe pump chamber 30 is reduced in thickness to that of the sealingwasher 34. This improves the self-priming ability of the pump. The pumpin FIG. 6 also has a minimally thick pump chamber 30. Further, the inlet22 and outlet 24 are perpendicular to the plane of the actuator 14, aconfiguration that may be more convenient in some applications.

In yet another embodiment, not shown, the bottom of the pump bodycomprises a piezoelectric actuator 14 arranged identically but as amirror image of the piezoelectric actuator 14 just described, with thesubstrate layers 40 facing each other across the pump chamber 30.

In still another embodiment, not shown, two of the pumps above describedare mounted side by side in one pump body. The actuator; seals; inletsand outlets, with one-way valve in the inlets only; pump covers; anddrivers are positioned in one or more of the configurations describedabove. In a preferred form of this embodiment, the drivers are in serieselectrically, with the pumps operating in parallel fluidwise in thesystem in which they are deployed.

INDUSTRIAL APPLICABILITY

This invention has particular application for water cooling of the CPUin computers but may have wider applications wherever a very small pumpof relatively high flow rate and minimum power consumption is needed tomove liquids at very low cost. The piezoelectric actuator by itself canhave very many other applications, such as speakers, audible alarms,automotive sensors, sound generators for active noise cancellation, andaccelerometers.

1. A method for making a piezoelectric actuator comprising: (1) coatingat least one of a first surface and a second surface of a piezoelectricelement with a polyimide adhesive; (2) heating the piezoelectric elementto dry the adhesive; then (3) inserting the piezoelectric elementbetween a first metallic layer and a second metallic layer to form anassembly; (4) placing the assembly in a press; (5) while the assembly isin the press, curing the polyimide adhesive at a curing temperaturewhich does not depole the piezoelectric element, thereby bonding thepiezoelectric element between the first metallic layer and the secondmetallic layer.
 2. The method of claim 1, wherein step (1) comprisescoating the piezoelectric with a thin layer of polyimide adhesive nomore than 0.005 inch thick.
 3. The method of claim 1, furthercomprising, as act (2) using heat to initiate a polymerization process.4. The method of claim 1, wherein the polyimide adhesive is a polyimidegel adhesive, and further comprising, as act (2) using heat to removesolvent from the polyimide gel adhesive.
 5. The method of claim 1,further comprising performing act (3) only after the polyimide adhesiveis dry to the touch.
 6. The method of claim 1, further comprising, atleast during act (5), applying uniform pressure to the piezoelectricactuator formed by bonding the piezoelectric element between the firstmetallic layer and the second metallic layer.
 7. The method of claim 6,wherein the act of applying pressure comprises: (a) laying a polyimidefilm on a bottom plate of a press; (b) placing the piezoelectricactuator on the polyimide film; (c) covering the piezoelectric actuatorwith a sheet of rubber; (d) tightening the press whereby the bottomplate and a top plate of the press apply uniform pressure to thepiezoelectric situated therebetween.
 8. The method of claim 6, furthercomprising: (e) cooling the piezoelectric actuator in the press; (f)removing the piezoelectric actuator from the press.
 9. The method ofclaim 1, wherein act (5) comprises inserting the assembly in the press,and then placing the press in a heater.
 10. The method of claim 1,further comprising using a polycrystalline material as the piezoelectricelement.
 11. The method of claim 1, further comprising using stainlesssteel as the first metallic layer.
 12. The method of claim 1, furthercomprising using aluminum as the second metallic layer.
 13. The methodof claim 1, further comprising forming the first metallic layer with arim arranged to serve as a clamping surface.
 14. The method of claim 13,further comprising forming the first metallic layer to have a largersurface area than the piezoelectric element.
 15. The method of claim 14,further comprising forming the second metallic layer to have a smallersurface area than the piezoelectric element.
 16. The method of claim 1,further comprising: forming the first metallic layer to have a largerdiameter than the piezoelectric element; and forming the second metalliclayer to have a smaller diameter than the piezoelectric element.
 17. Themethod of claim 1, further comprising using a flat element as thepiezoelectric element.
 18. The method of claim 1, further comprisingusing as the polyimide adhesive as a gel which has a minimum of 25%solids.
 19. The method of claim 1, wherein the curing temperature doesnot melt the polyimide adhesive.
 20. The method of claim 1, wherein thecuring temperature is no more than 200 degrees Centigrade.